Packaging films and heavy duty bags/sacks for various commercial and consumer applications may be produced from various polymers and their blends. Such films can be manufactured using either monolayer or co-extrusion processes. Polyethylene resins made with different catalysts, manufacturing technologies and operating conditions provide different molecular characteristics and performance attributes. Commonly used polyethylene resins in the packaging industry are broadly characterized as linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), low density polyethylene (LDPE) and high density polyethylene (HDPE). These types of polyethylene resins and their blends are commonly used to manufacture flexible films, sheets and bags for different needs and applications. In some cases, polypropylene and/or other materials can also blended to modify the end use characteristics, e.g. to increase the stiffness of the heavy duty sack films.
Some performance properties of flexible films include film toughness (dart impact strength), machine and transverse direction (MD and TD) tear strengths, film stiffness (or secant modulus), tensile yield strength, puncture resistance and sealability (seal initiation temperature and maximum seal strength). Creep resistance is another performance attribute useful for heavy duty sacks/bags and for form-fill-seal packaging for various applications, e.g. packaging of resins, fertilizers, cotton, salt, stones, lawn and garden supplies, insulation, building materials, cement, pet foods, flour, seed and feed etc. Furthermore, the film manufacturers prefer good resin processability with reasonable extruder current and pressures, and good bubble stability to achieve higher outputs and plant productivity. In automatic packaging operations, such as form-fill-seal packaging, it is desired to hold the contents of the package without excessive creep or wrinkling especially when the contents of the package are hotter than the film.
Thus, there exists a need for new ethylene copolymer architectures that can provide improved creep resistance and high toughness and a good balance of film stiffness and processability in monolayer and multi-layer film structures.
A need exists for single site catalyzed ethylene copolymers having high film toughness properties that are relatively easy to process or convert into finished product. Furthermore, a need exists for an ethylene copolymer that exhibits high creep resistance and good processability. Although not wishing to be bound by theory, a uniform melting/freezing behavior of an ethylene copolymer (as exhibited by a single peak in the differential scanning calorimeter, DSC, measurement) may help in co-crystallization of different sized polymer molecules without much segregation thereby improving the creep resistance of films. Therefore, a need exists for an ethylene copolymer that has uniform melting behavior, i.e. exhibiting a single peak in a Differential Scanning Calorimeter (DSC) measurement.